Investigation of the reliability of complex systems is an important part of the design and production process. To obtain reliable results about reliability it is not sufficient just to perform tests but it is also necessary to plan a test program carefully and also to analyse test results in a way so that statistically reliable results can be obtained.
Such a method generally comprises steps of: (a.) establishing a system hierarchy of several levels by identifying components of the system; (b.) identifying and assigning failure modes to components; (c.) determining test procedures for failure modes of different components to obtain estimates for the availability of the component; and (d.) determining test procedures for failure modes of different components to obtain estimates for the availability of the component.
In establishing a system hierarchy of several levels by identifying components of the system. A system is a complex technical entity like a machine, a vehicle or the like consisting of several parts, here designated as components. Components may consist of parts themselves, so that a hierarchy is established in which the system consist of several components of a first level. At least some of these first level components consist of components of a second level and so forth. Here the term subcomponent is used for a part of a component of a higher level. In this step, the system is analysed and a tree of components is identified, representing the system hierarchy.
Identifying and assigning failure modes to components. All components are analysed to detect possible failures which may cause a malfunction of the system causing unavailability. These failure modes are described in all their aspects to get comprehensive information including: (i.) kind of failure, e.g., wear, (ii.) location of failure, e.g., sliding surface, cause-effect-chain, e.g., high local pressure, high temperatures, partial melting of surface, adhesive wear, crack formation, (iii.) observable stress parameters, and (iv.) precursors.
Usually some components may have several failure modes, some of them only one. It may occur that a certain component has no failure mode assigned, but in this case at least one subcomponent should have a failure mode assigned.
Another very important parameter connected with each failure mode is the expected downtime of the system in the case of such failure, including time for detection, delivery of spare parts and repair.
Availability is the percentage of operation time the component is working. Any failure occurring results in a downtime until a successful repair is finished and this downtime represents the unavailability of the system.
Determining test procedures for failure modes of different components to obtain estimates for the availability of the component. At least one test is selected. In prior art solutions these tests are designed in a way that it can be shown by statistical calculations that a predetermined lifetime can be achieved. In detail a sufficient duration of the test is determined which can be compared to a certain time of consumer operation (equivalent duration).
The disadvantage of such procedure is that tests have usually to be designed in a special way to obtain a prognosis of lifetime or availability and that in complicated cases some tests may be too stringent. At the same time it is possible to overlook potential risks.